Phenacetin O-deethylation by human liver microsomes in vitro: Inhibition by chemical probes, SSRI antidepressants, nefazodone and venlafaxine

Biotransformation of phenacetin via O-de-ethylation to acetaminophen, an index reaction reflecting activity of Cytochrome P450-1A2, was studied in microsomal preparations from a series of human livers. Acetaminophen formation was consistent with a double Michaelis-Menten system, with low-K-m (mean K-m1 = 68 mu M) and high-K-m (mean K-m2 = 7691 mu M) components. The low-K-m enzyme accounted for an average of 96% of estimated intrinsic clearance, and was predicted to contribute more than 50% of net reaction velocity at phenacetin concentrations less than 2000 mu M. Among index inhibitor probes, alpha-naphthoflavone was a highly potent inhibitor of the low-K-m enzyme (K-i1 = 0.013 mu M); furafylline also was a moderately active inhibitor (K-i1 = 4.4 mu M), but its inhibiting potency was increased by preincubation with microsomes. Ketoconazole was a relatively weak inhibitor (K-i1 = 32 mu M); quinidine and cimetidine showed minimal inhibiting activity. Among six selective serotonin reuptake inhibitor (SSRI) antidepressants, fluvoxamine was a potent inhibitor of 1A2 (mean K-i1 = 0.24 mu M). The other SSRIs were more than tenfold less potent. Mean K-i1 values were: fluoxetine, 4.4 mu M; norfluoxetine, 15.9 mu M; sertraline, 8.8 mu M; desmethylsertraline, 9.5 mu M; paroxetine, 5.5 mu M. The antidepressant nefazodone and four of its metabolites (meta-chloro-phenylpiperazine, two hydroxylated derivatives, and a triazoledione) were very weak inhibitors of P450-1A2. Venlafaxine and its O- and N-desmethyl metabolites showed minimal inhibitory activity.